The scale on a map is 10cm : 6km. If the distance between two cities is 42km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 10cm on the map corresponds to an actual distance of 6km. An actual distance of 42km is the same as 7 $\cdot$ 6km. The distance between the two cities on the map is 7 $\cdot$ 10cm, or 70cm.